The present invention relates to heating, ventilating and air conditioning ("HVAC") duct and, more specifically, to an integrally reinforced rectangular duct system and to the related improved method for reinforcing rectangular duct.
It is well-known that longer and larger cross-section HVAC ducts require transverse reinforcement spaced along the lengths thereof. The specific reinforcement requirements, which depend on many factors including joint type, system air pressure, gage of material and finished duct size, are set by industry standards and building codes, for example, as found in the manual HVAC Duct Construction Standards, Metal & Flexible published by SMACHNA, the Sheet Metal and Air Conditioning Contractors' National Association, Inc. headquartered in Merrifield, Va. Further consideration of the specific SMACHNA standards is beyond the scope of this application. Rather, the present invention pertains to a system of duct reinforcement when such reinforcement, in accordance with SMACHNA or other applicable standards, is required.
For the present it is sufficient to observe that duct reinforcement may be defined either at the ends of a duct section--in the form of duct section connecting joints--or at spaced intermediate locations along the length of a given duct section. To the extent that short duct sections are employed, with the corresponding increase in the number of `inherently-reinforced` joints, additional, intermediate reinforcement may not be required.
Unfortunately, the cost of such joints and of the labor to fabricate and assemble them, significantly limits the efficacy of the `joint` approach to duct reinforcement. The present invention, therefore, is directed to an economical system for intermediate duct reinforcement that facilitates use of longer and larger ducts and/or of ducts fabricated from less expensive lower gage sheet metal stock.
Many forms of intermediate duct reinforcement have been proposed and are in common use today including several shown in the previously noted SMACHNA manual. But without exception each known system is either time or material intensive --often requiring substantial labor, first, to fabricate the reinforcement assemblies, thereafter, to position and secure the assemblies to the duct.
Probably the most common form of reinforcement is conventional `angle`, `zee`, or `channel` iron, which iron is pre-fabricated into rectangular reinforcement loops or brackets then positioned around the duct transversely to the direction of air flow. See, for example, U.S. Pat. No. to Godshalk, No. 3,202,184. A brief review of the steps required to implement this prior art system of reinforcement quickly reveals the shortcomings of the approach. First, appropriate angle must be inventoried. And while inventorying may seem trivial, it involves myriad activities including, first, monitoring inventory levels and purchasing additional stock as required. Labor and the possibility of oversight, i.e. the failure to timely order needed angle, are inherent.
Next, the uncut angle stock is delivered in relatively long and heavy sections that must be unloaded (more labor) and stored in appropriate racks (using shop space too often already in short supply). These steps represent just the beginning.
Actual fabrication is at least as labor intensive. First, the ungainly angle must again be handled, measured and severed into appropriate sections--generally four per reinforcement assembly--and the unused angle returned to storage. Holes are generally required in the angle for assembly and .installation of the reinforcement member. Further, reinforcement members are typically `preassembled` prior to installation onto the duct and therefore the four angle pieces must be positioned and aligned--usually on a jig--thereafter, bolted and/or welded together. Finally the preassembled reinforcement member must be slid into position on the duct and appropriately affixed thereto.
A further limitation of the above-described angle system of reinforcement relates to whether the duct is to be shipped to the job site completely fabricated or `knocked down`, that is, to what extent must field, as opposed to shop, personnel be employed to install the reinforcement assemblies. (It is generally preferable to limit on-site duct fabrication to final assembly and installation of the `knocked down` duct.) In this connection it will be appreciated that shipment of completed rectangular duct may be impractical in view of the large size (volume) of such completed duct--one ships mostly `air` in this assembled configuration. For this reason, duct is generally shipped `knocked down` and nested. And as a consequence, the preassembled, rectangular angle reinforcement assembly must generally be field installed.
Another well-known reinforcement scheme which as above, also requires substantial on-site installation is the `tie rod`. Tie rods are, literally, rods positioned inside the duct and attached between parallel side panels to preclude duct-ballooning or `oil-canning` as it is known in the trade. Tie rods work well, but again reflect high fabrication/installation labor particularly, where the rods are secured by the welding of appropriate brackets to the interior duct wall surfaces.
In view of the foregoing, it is not surprising that less labor-intensive reinforcing solutions have been proposed. One such solution is the formed sheet metal reinforcement of Greiner, U.S. Pat. No. 4,621,661, Greiner is directed to solving the same problem herein described and, indeed, Greiner confirms that it is often less expensive to make a particular piece of duct from a heavier gage material if such increase obviates the need for intermediate reinforcement.
Greiner proposes a pair of stiffeners oriented transversely across the two sides of a rectangular duct (the long sides), the stiffeners being formed from sheet metal and affixed to the duct using a punch/die tool that places successive `dimples` along the length of the stiffener and through the several layers of sheet metal (layers comprising both duct and stiffener). Unquestionably Greiner performs a `stiffening` function and, in so doing, minimizes certain of the above-noted problems, for example, the need to order and handle angle stock. But Greiner does not disclose a full circumferential reinforcement system (i.e. four sides) nor the mechanism for interconnecting the corners of such a system. Further, Greiner still requires the processing and handling of separate reinforcement members. And probably the greatest obstacle to industry acceptance of the Greiner system has been the cost of the `dimple` machinery and the cumbersomeness in handling and positioning the sheet metal pieces during the dimpling process.
The present invention relates to a method and structure for transversely reinforcing rectangular duct--circumferentially around all four sides of the duct--and without the need to inventory, handle or `pre-fabricate` angle iron or other reinforcing material. Indeed, the present invention defines an integral reinforcement rib whereby the reinforcer, itself, is fabricated of sheet metal material but, unlike Greiner, the requisite rib material is defined within, and as part of, the sheet metal blank(s) that comprise the completed fitting. As a consequence of this construction, the entire fitting, including the reinforcement ribs, may be fabricated through a series of roll-forming or similar automated procedures.
Substantially all fabrication of the duct, including its integral reinforcement ribs, is performed in the shop with the duct, thereafter, being shippable in `knocked down` form to the job site. All that is required on-site is the placement of standard, mass-produced `rib corners` onto and at the corners of the integral duct reinforcement ribs.
It is customary to fabricate straight duct sections in lengths corresponding to the width of coil stock available in order to minimize material waste (e.g. 4' or 5'). The same is true for the present integrally reinforced duct system, although a small portion of this width must be `ear-marked` for the integral rib which, consequently, results in a slightly shorter finished duct section. The present duct may be fabricated, in the same manner as conventional duct, from a single wrap-around blank or from L-shaped or other conventional multi-blank topologies.
Most typically the present duct will incorporate a single transverse reinforcement rib (i.e. circumferentially around all four duct sides or across the planar sides of flat-oval duct). positioned midway along the length of the duct. For duct fabricated from five foot coil stock, the rib would be positioned approximately 30 inches from the ends thereof.
It will be appreciated that the amount of material `dedicated` to the reinforcement rib, and therefore the height of the rib itself may be selected according to the reinforcement mandated--the higher the rib, the more reinforcement strength provided. A rib height may advantageously be selected to correspond to the dimensions of the end joints, for example, 13/8" for the "TDC" joint manufactured by The Lockformer Company/Iowa Precision Industries, Inc.
According to a preferred embodiment of the present duct, the rib will be automatically machine fabricated by auto-braking or roll-forming each duct sheet thereby transforming the so-called `dedicated` portion of the sheet into a rib, extending perpendicularly from the duct surface, and comprised of upwardly and downwardly folded metal, i.e. a rib of double thickness. The `dedicated` portion of the blank(s) is the substantially rectangular region defined across the entire width of each blank, midway between the opposed ends and of length equal to twice the intended rib height. Thus, as noted, the overall length of the finished duct section will be correspondingly shortened by this length of `dedicated` material, i.e. again, by twice the height of the rib.
Conventional rectangular duct blank(s) cannot be directly utilized in conjunction with the integral reinforcement of the present invention. Additional steps of `blanking` relief slots and notches in the `dedicated` portion of the duct blank along the seam edges and intended fold lines are required to permit subsequent duct fabrication. Fortunately these additional steps may be automated and performed on tooled punch equipment or, alternatively, as part of the original blank cutting program (e.g. utilizing computer aided design and computer-controlled plasma cutting apparatus) and therefore such steps represent, at most, only trivial added labor elements.
The importance of these relief slots/notches cannot be Understated as the very integrity and reinforcing function of the integral rib would otherwise preclude the bending required, for example, to fabricate the two L-shaped components of this popular duct topology. See by way of contrast the drainage culvert of Smith, U.S. Pat. No. 1,057,098, in which a relatively low rib is, and in the absence of the present relief slots must be, deformed to create the desired circular culvert contour. Further, the seam material in the `dedicated` portion of the blank must especially be removed by reason that the seam--which is ordinarily fabricated as a multi-layer interface (e.g. a Pittsburgh)--will preclude rib roll-forming or, at the least, will result in a misformed bulbous rib of four or more layers.
One further step contemplated to properly implement the integral rib of the present invention is the spot welding or other securement of the dual-thickness ribs to assure the integrity of these ribs, i.e. that the ribs surfaces do not separate, under ordinary loads. Again, however, this securement step may include `dimpling` or similar affixation as part of the rib roll-forming process or, at least, as an added integral station to the roll-forming line. As such, this step does not significantly diminish nor detract from the low-labor/low-cost cornerstones of the present technology.
It will be noted that the above-described fabrication is both automated and shop-based. The duct sub-sections, e.g. the L-shaped duct components, may be nested and shipped `knocked down` to the job site for final assembly and installation per standard practice. Very little has to be performed on the job site in connection with the reinforcement of the present invention.
The single exception is the installation of the rib corners--which installation generally follows the final on-site assembly of the duct. But even here, little complexity or labor is involved. The rib corners are in the first instance designed for inexpensive, mass production and therefore are available at the job-site as purchased, no-labor items. Advantageously, and to the extend /that the present invention is implemented utilizing a single or limited number of rib heights, a standard `one-size-fits-all` corner can be used on all reinforced ducts, regardless of duct cross-section.
A principal function of the rib corner is to seal the ends of the rib against air leakage. (Leakage is generally not a problem associated with `joint` corners by reason that the full transverse seal required at joints precludes entry of air into the joints and joint corners.) The rib corner may perform a secondary `strengthening` function by reason of its rigid attachment to the pair of ribs that converge to near-intersection at each duct corner. In any event, the rib corner is preferably provided with sealing material therein, e.g. closed-cell neoprene, and designed for `slip-on` attachment over the reinforcement ribs. It is contemplated that a dimpling tool or `twist-lock` tool, as more folly described below, may advantageously be employed to efficaciously lock the corner in position consistent with the low-cost/labor objectives of the present invention.
It is therefore an objective of the present invention to provide a method and structure for duct reinforcement that is both efficacious in performance and inexpensive in construction taking into consideration all of the costs associated therewith including material, material handling, fabrication and assembly, both in-shop and on-site. To this end it is a further object to minimize use of separate reinforcing material, particularly material that requires special inventorying, handling and prefabrication. It is an object to utilize standard sheet metal for duct reinforcing and to make the reinforcement integral to the duct. It is a further object to minimize on-site labor in connection with the reinforcement of duct and therefore to the extent that post-shop fabrication is required it is an object to employ standardized and/or prefabricated components that may be field-installed without significant further fabrication or installation time.